In spite of extensive medical research and numerous advances, cancer remains the second leading cause of death in the United States. Hepatocellular carcinoma (HCC or malignant hepatoma) is one of the most common cancers in the world, and is especially problematic in Asia.
Treatment prospects for patients with hepatocellular carcinoma are dim. Even with improvements in therapy and availability of liver transplant, only a minority of patients are cured by removal of the tumor either by resection or transplantation. For the majority of patients, the current treatments remain unsatisfactory, and the prognosis is poor.
Of particular interest is development of more specific, targeted forms of cancer therapy, especially in cancers that are difficult to treat successfully, such as hepatoma. In contrast to conventional cancer therapies, which result in relatively non-specific and often serious toxicity, more specific treatment modalities attempt to inhibit or kill malignant cells selectively while leaving healthy cells intact.
One possible treatment approach for cancers such as hepatoma is gene therapy, whereby a gene of interest is introduced into the malignant cell. The gene of interest may encode a protein which converts into a toxic substance upon treatment with another compound, or an enzyme that converts a prodrug to a drug. For example, introduction of the herpes simplex gene encoding thymidine kinase (HSV-tk) renders cells conditionally sensitive to ganciclovir (GCV). Alternatively, the gene of interest may encode a compound that is directly toxic, such as diphtheria toxin (DT). For these treatments to be rendered specific to cancer cells, the gene of interest can be under control of a transcriptional initiation region that is specifically (i.e., preferentially) activated in the cancer cells. Cell or tissue specific expression can be achieved by using cell-specific enhancers and/or promoters. See generally Huber et al. (1995) Adv. Drug Delivery Reviews 17:279–292.
A variety of viral and non-viral (e.g., liposomes) vehicles, or vectors, have been developed to transfer these genes. Of the viruses, retroviruses, herpes simplex virus, adeno-associated virus, Sindbis virus, poxvirus, and adenoviruses have been proposed for gene transfer with retrovirus vectors or adenovirus vectors being the focus of much current research. Adenoviruses are among the most easily produced and purified, whereas retroviruses are unstable, difficult to produce and to purify, and may integrate into the host genome, raising the possibility of dangerous mutations. Moreover, adenovirus has the advantage of effecting high efficiency of transduction and does not require cell proliferation for efficient transduction of cell. For general background references regarding adenovirus and development of adenoviral vector systems, see Graham et al. (1973) Virology 52:456–467; Takiff et al. (1981) Lancet 11:832–834; Berkner et al. (1983) Nucleic Acid Research 11: 6003–6020; Graham (1984) EMBO J 3:2917–2922; Bett et al. (1993) J. Virology 67:5 911–5921; and Bett et al. (1994) Proc. Natl. Acad. Sci. USA 91:8802–8806.
When used as gene transfer vehicles, adenovirus vectors are often designed to be replication-defective and are thus deliberately engineered to fail to replicate in the target cells of interest. In these vehicles, the early adenovirus gene products E1A and/or E1B are deleted and provided in trans by the packaging cell line 293. Graham et al. (1987) J. Gen. Virol 36:59–72; Graham (1977) J. Genetic Virology 68:937–940. The gene to be transduced is commonly inserted into adenovirus in the E1A and E1B region of the virus genome. Bett et al. (1994). Replication-defective adenovirus vectors as vehicles for efficient transduction of genes have been described by, inter alia, Stratford-Perricaudet (1990) Human Gene Therapy 1:241–256; Rosenfeld (1991) Science 252:431–434; Wang et al. (1991) Adv. Exp. Med. Biol. 309:61–66; Jaffe et al. (1992) Nat. Gen. 1:372–3 78; Quantin et al. (1992) Proc. Natl. Acad. Sci. USA 89:2581–2584; Rosenfeld et al. (1992) Cell 68:143–155; Stratford-Perricaudet et al. (1992) J. Clin. Invest. 90:626–630; Le Gal Le Salle et al. (1993) Science 259:988–990 Mastrangeli et al. (1993) J. Clin. Invest. 91:225–234; Ragot et al. (1993) Nature 361:647–650; Hayaski et al. (1994) J. Biol. Chem. 269:23872–23875; Bett et al. (1994).
The virtually exclusive focus in development of adenoviral vectors for gene therapy is use of adenovirus merely as a vehicle for introducing the gene of interest, not as an effector in itself. Replication of adenovirus has been viewed as an undesirable result, largely due to the host immune response. In the treatment of cancer by replication-defective adenoviruses, the host immune response limits the duration of repeat doses at two levels. First, the capsid proteins of the adenovirus delivery vehicle itself are immunogenic. Second, viral late genes are frequently expressed in transduced cells, eliciting cellular immunity. Thus, the ability to repeatedly administer cytokines, tumor suppressor genes, ribozymes, suicide genes, or genes which convert prodrug to an active drug has been limited by the immunogenicity of both the gene transfer vehicle and the viral gene products of the transfer vehicle as well as the transient nature of gene expression. There is a need for vector constructs that are capable of eliminating essentially all cancerous cells in a minimum number of administrations before specific immunological response against the vector prevents further treatment.
A completely separate area of research pertains to the description of tissue-specific regulatory proteins. α-Fetoprotein (AFP) is an oncofetal protein, the expression of which is primarily restricted to developing tissues of endodermal origin (yolk sac, fetal liver, and gut), although the level of its expression varies greatly depending on the tissue and the developmental stage. AFP is of clinical interest because the serum concentration of AFP is elevated in a majority of hepatoma patients, with high levels of AFP found in patients with advanced disease. The serum AFP levels in patients appear to be regulated by AFP expression in hepatocellular carcinoma but not in surrounding normal liver. Thus, the AFP gene appears to be regulated to hepatoma cell-specific expression.
The 5′ upstream flanking sequence of the human AFP gene has been shown to confer cell-specific enhancer activity. Watanabe et al. (1987) J. Biol. Chem. 262:4812–4818; see also Sakai et al. (1985) J. Biol. Chem. 260:5055–5060 (describing cloning the human AFP gene). Canadian pat. appl. No. 2,134,994. An enhancer is a cis-acting transcriptional regulatory element known to play a major role in determination of cell-specificity of gene expression. The enhancer is also typically characterized by its ability to augment transcription over a long distance and relatively independently of orientation and position with respect to its respective gene. A promoter is located immediately 5′ (upstream) of the transcription start site and generally includes an AT-rich region called a TATA box.
Several approaches for gene therapy using the cell-specific AFP enhancer to treat hepatoma have been described. Tamaoki and Nakabayashi describe using the AFP transcriptional regulatory regions to drive expression in AFP-producing cells, particularly linking a gene encoding a cancer cell toxin to the AFP transcriptional regulatory region. Canadian pat. app. No. 2,134,994. However, the entire focus of this publication was that of expression of a heterologous toxin gene, such as the gene encoding diphtheria toxin (DT), and adenovirus was only described in terms of a delivery vehicle for this toxin gene. Kaneko et al. and Kanai et al. describe adenovirus-mediated gene therapy of hepatoma using the 5′ upstream region of AFP to restrict HSV-tk gene expression to hepatocellular carcinoma cells, followed by treatment with nucleoside analog GCV. Cancer Res. 55:5283–5287 (1995); Hepatology 22 (4 Part 2): Abstract 158A (1995); Hepatology 23:1359–1368 (1996); Hepatology 22:Abstract 328 (1995). However, these adenovirus constructs are replication defective, and the entire focus of these publications is using the AFP 5′ upstream transcriptional regulatory region to control expression of a non-adenovirus gene. Wills et al. (1995) also describe replication-deficient adenoviral vectors which selectively express HSV-tk. Cancer Gene Ther. 2:191–197. Kanai et al. (1996) also reported using the AFP enhancer-promoter to drive expression of the lacZ gene and the E. coli cytosine deaminase (CD) gene in addition to the HSV-tk gene. Gastroenterology (Supp) 110:A1227. Again, the focus and approach entailed using replication-deficient adenovirus as a therapeutic gene delivery vehicle, not as an agent per se for effecting selective growth inhibition. See also Arbuthnot et al. (1996) (describing using 5′ flanking sequences from rat AFP gene). Human Gene Ther. 7:1503–1514.
Hepatocellular carcinoma is rarely curable by standard therapies. Thus, it is critical to develop new therapeutic approaches for this disease. The present invention addresses this need by providing adenoviral vectors specific for replication in AFP-producing cells.
All publications cited herein are hereby incorporated by reference in their entirety.